Sub-half micron multilevel metallization is one of the key technologies for the next generation of very large scale integration ("VLSI"). The multilevel interconnections that lie at the heart of this technology require planarization of interconnect features formed in high aspect ratio apertures, including contacts, vias, lines or other features. Reliable formation of these interconnect features is very important to the success of VLSI and to the continued effort to increase circuit density and quality on individual substrates and die.
One means for increasing circuit density is to decrease the dimensions of the metal conductors that make up the integrated circuit. As the dimensions are made smaller, the operating speed increases and the power density remains constant, but the current density is increased in proportion to the scale-down factor. Metal conductors have an upper current density limit imposed by electromigration. Electromigration is a diffusive process in which the atoms of a solid move from one place to another under the influence of electrical forces. This effect limits the maximum current that can be carried by a conductor without its rapid destruction. For example, the current density for aluminum conductors of integrated circuits must be kept lower than 10.sup.6 A/cm.sup.2. Electromigration does not limit the minimum device size but, rather, limits the number of circuit functions that can be carried out by a given number of connected circuit elements per unit time. Highly oriented crystalline growth of the conducting layers have enhanced electromigration resistance. Therefore, as geometries of integrated circuits are reduced, the need for highly oriented films increases. Ideally, a film layer having a &lt;111&gt; crystal orientation is formed on the substrate to improve the electromigration properties of the film at these small geometries.
Two conventional methods for depositing film layers is by chemical vapor deposition ("CVD") and physical vapor deposition ("PVD"). CVD processes typically include a blanket process and a selective process wherein the deposition of a film layer occurs when a component of the chemical vapor contacts a "nucleation site" on the substrate. The component attaches to the nucleation site, creating a deposit surface on which further deposition proceeds. In a blanket CVD process, all surfaces serve as nucleation surfaces, and the vapor will deposit a film on the entire exposed surface of the substrate, including the side and bottom surfaces of an aperture as well as the field. A selective process typically deposits a film only on select nucleation sites provided on the substrate, typically at the base of apertures.
Thin films deposited during a blanket CVD process are usually conformal and provide excellent step coverage, i.e., uniform thickness of layers on the sides and base of any aperture formed on the substrate, even for very small aperture geometries. Therefore, blanket CVD is a common method used to fill apertures. However, there are two primary difficulties associated with blanket CVD processes. First, blanket CVD films grow from all sides in an aperture which typically results in a void in the filled aperture because the deposited layer grows upwardly and outwardly at the upper corners of the aperture and bridges at the upper surface of the aperture before the aperture has been completely filled (i.e., "crowning"). Also, a nucleation layer, i.e., a continuous film layer to insure nucleation over all surfaces of the substrate, which must be deposited on the aperture walls to ensure deposition of the CVD layer thereon, further reduces the width of the aperture, thereby increasing the difficulty of void-free filling of the aperture without voids. Second, films deposited by blanket CVD tend to conform to the topography of the surface on which the films are deposited which may result in a film having a randomly oriented crystal structure and resulting lower reflectivity properties and poor electromigration performance if the topography is non-oriented or random.
Selective CVD is based on the fact that the decomposition of the CVD precursor gas to provide a deposition film usually requires a source of electrons from a conductive nucleation film. In accordance with a conventional selective CVD process, deposition should occur in the bottom of an aperture where either a conducting film or doped silicon from the underlying layer has been exposed, but should not grow on the insulative field or insulative aperture walls where no nucleation sites are provided. These conducting films and/or doped silicon exposed at the base of the apertures, unlike dielectric surfaces, supply the electrons needed for decomposition of the precursor gas and resulting deposition of the film layer. The result obtained through selective deposition is a "bottom-up" growth of the film in the apertures capable of filling very small dimension (&lt;0.25 .mu.m), high aspect ratio (&gt;5:1) vias or contacts. However, in selective CVD processes unwanted nodules form on the field where defects in that surface exist.
PVD processes, on the other hand, enable deposition of highly oriented films having improved reflectivity, but do not provide good aperture filling or step coverage in high aspect ratio applications. Physical sputtering of target material results in particles traveling at acute angles relative to the substrate surface. As a result, where high aspect ratio apertures are being filled, sputtered particles tend to deposit on the upper wall surfaces and cover the opening thereof before the aperture is completely filled with deposition material. The resulting structure typically includes voids therein which compromise the integrity of the devices formed on the substrate.
High aspect ratio apertures can be filled using PVD processes by depositing the film at elevated temperatures. As an example, aluminum can be deposited at 400.degree. C. or higher to enhance flow of the aluminum on the surface and throughout the aperture. It has been found that this hot Al process provides improved step coverage. However, a film deposited using a hot Al process has been shown to have poor reflectivity. High reflectivity is an important characteristic in films because it is an indication of a highly oriented crystal structure which means better electromigration performance and enables better definition of lines in a photolithographic processes which are used to pattern the layers formed on the substrate during integrated circuits fabrication.
Therefore, there is a need for a metallization process for void-free filling of apertures, particularly high aspect ratio, sub-quarter micron applications which reduces the problems of nodule formation on the field and provides uniform growth of a film layer on the field while the aperture is being filled. More particularly, it would be desirable to have a process to accomplish selective deposition within high aspect ratio sub-quarter micron apertures and simultaneous blanket deposition of a highly oriented (i.e., &lt;111&gt;) film on the field, particularly if the process formed highly oriented films at a controllable rate. There is also a need to provide enhanced reflectivity of metal films deposited by PVD or CVD techniques over barrier or liner layers. It would be advantageous to find a single process which can be used in one application to eliminate nodule formation and improve selectivity and in another application to provide films with improved reflectivity.